This invention relates to a second harmonic wave generating device having an extremely high conversion efficiency to second harmonic wave, more specifically, to a second harmonic wave generating device (hereinafter referred to as "SHG device") comprising a LiTaO.sub.3 thin film formed on a LiNbO.sub.3 substrate and a LiNbO.sub.3 thin film waveguide layer formed on the LiNbO.sub.3 thin film.
A SHG device utilizes nonlinear optical effects of a nonlinear optical material to convert wavelength .lambda. of incident laser light to wavelength 1/2.lambda., which is outputted. Since the output light has 1/2 the wavelength of incident light, the device can be used in an optical disc memory and CD player to achieve a 4-times increase in recording density, and can be used in a laser printer and photolithography with enhanced resolution.
Heretofore, a bulk single crystal of a nonlinear optical material using a high-output-power gas laser as a light source has been used as a SHG device. However, with recent increases in demand for compact optical disc systems and laser printers and since gas laser requires an external modulator for optical modulation and is not suited for compact design, a SHG device that enables and use of a semiconductor laser, which can be directly modulated and is lower in cost and easier to handle than gas laser, has been in demand. When a semiconductor laser is used as a light source, since the semiconductor laser generally has a low output power of several mW to several ten mW, a SHG device of a thin film waveguide structure which has a particularly high conversion efficiency has been required.
Optical materials that have nonlinear optical effects and can be used in a SHG device include lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3), KTiOPO.sub.4, KNbO.sub.3, Ba.sub.2 NaNb.sub.5 O.sub.15, and Ba.sub.2 LiNb.sub.5 O.sub.15 and, among these, LiNbO.sub.3 is high in nonlinear optical constant and low in optical loss, which makes it most suitable for use in a SHG device.
Heretofore, there have been known methods to form an optical waveguide using LiNbO.sub.3, in which a bulk single crystal of LiNbO.sub.3 is treated by Ti diffusion, proton exchange, or out-diffusion to form layers with different refractive indices. However, waveguides obtained using these methods have problems in that it is difficult to have great differences in refractive index from the bulk crystal (with respect to short wavelength laser light especially not more than 1 .mu.m in wavelength), it is also extremely difficult to make phase matching for the generation of the second harmonic wave, that is, to conform the refractive index in the waveguide (effective index) of incident light to the effective index of the second harmonic wave [Yamada and Miyazaki, Technical Report of the Electronic Information Communication Society, MW87-113 (1988)], and it is difficult to obtain a high conversion efficiency because, since the boundary between the waveguide and the bulk crystal is not well defined, the light wave tends to diffuse from the waveguide and it is thus difficult to concentrate the optical energy.
The inventors, using a LiNbO.sub.3 material and a 0.8 .mu.m-band semiconductor laser light source, have conducted investigations for a combination that enables phase matching between the fundamental light wave and the second harmonic wave, and found a condition that enables phase matching using a structure comprising a LiTaO.sub.3 single crystal having a specific refractive index and a LiNbO.sub.3 thin film as a waveguide layer having a specific refractive index formed on the surface of the LiTaO.sub.3 single crystal, thus accomplishing a previous invention, which was applied for a patent as Japanese Patent Application No. 63-160804/1988.
However, the previous invention was of a type with a very narrow range of application which was able to use only a 0.8 .mu.m-band semiconductor laser as a fundamental laser light source. Furthermore, since refractive index of a substance generally varies with wavelength of light applied, the previous invention could not be applied to laser light sources of different wavelengths, and in prior art LiNbO.sub.3 thin film is directly formed on LiTaO.sub.3 substrate.
Commercial LiTaO.sub.3 single crystal substrates used as carriers for single crystal thin films are for use in SAW devices, which are high in impurity content (&gt;2 ppm), large in refractive index distribution (10.sup.-3 /cm), and inferior in crystallinity. Therefore, when used as substrates for optical thin films, since thin film waveguide layer formed on the substrates transfers the crystallinity of the substrate, these substrates tend to cause defects such as domains, which lead to deterioration in characteristics such as optical transmission, electrooptical effect and nonlinear optical effects.
Furthermore, although it is possible to use an optical-grade LiTaO.sub.3 substrate, optical-grade single crystal substrates are very little commercialized except for LiNbO.sub.3, are thus expensive, and not suitable for general-purpose use.
The inventors have conducted intensive studies into conditions for the use of a LiNbO.sub.3 material for a SHG device which enables phase matching with laser light sources of different wavelengths. As a result, it has been newly found that a second harmonic wave can be generated very efficiently by using a substrate comprising a LiTaO.sub.3 thin film formed on a LiNbO.sub.3 substrate, forming a LiNbO.sub.3 thin film waveguide layer on the LiTaO.sub.3 thin film, setting an ordinary refractive index (n.sub.oS1) at a fundamental wavelength (.lambda..mu.m) and an extraordinary refractive index (n.sub.eS2) at a second harmonic wavelength (.lambda..mu.m/2) of the LiTaO.sub.3 thin film to specific ranges, and setting an ordinary refractive index (n.sub.oF1) of the LiNbO.sub.3 thin film waveguide layer at the fundamental wavelength (.lambda..mu.m), the extraordinary refractive index (n.sub.eS2) of the LiTaO.sub.3 thin film at the second harmonic wavelength (.lambda..mu.m/2), and an extraordinary refractive index (n.sub.eF2) of the LiNbO.sub.3 thin film waveguide layer at the second harmonic wavelength (.lambda..mu.m/2) so as to satisfy a specific relationship, thus accomplishing the present invention.
Furthermore, the inventors have also found that in adjusting the thickness of the thin film waveguide layer, prior art optical polishing tends to cause a deterioration of crystallinity on the polished surface, and dry etching is most suitable as a processing method that is able to prevent deterioration of crystallinity of such a polished surface, thus accomplishing the present invention.